This invention relates generally to mode-locked lasers. More particularly, it relates to enhancement of saturable Bragg reflectors (SBRs) using a back side of the SBR substrate.
The type of passive mode-locker that has had the greatest recent impact is based on saturable absorption in semiconductor films. One type of saturable absorber device is know as a saturable Bragg reflector (SBR). An SBR generally comprises a Bragg stack disposed on a substrate. The Bragg stack typically contains alternating layers of narrow bandgap, e.g. Gallium Arsenide (GaAs), and wide bandgap, e.g. Aluminum Arsenide (AlAs), layers with a saturable absorber layer inserted in one of the narrow bandgap layers. The layers are usually deposited by metallo-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). An alternative to semiconductor saturable absorbers formed by epitaxial growth (MOCVD, MBE) is to use films formed by simpler deposition methods (sputter, evaporation) that are commonly used for coating optical components. [Bilinsky, 1998] This approach is much less well developed at this time compared to the semiconductor material. The reflection characteristics of the SBR generally depend on the wavelength of the incident radiation. The saturable layer modifies the reflection characteristics of the SBR near a characteristic exciton wavelength. The absorber layer within the Bragg stack provides a non-linear reflectance as a function of fluence. The reflectance of such a stack tends to increase with increasing fluence and saturates as the fluence rises above a predetermined level. The reflectance is affected by an exciton effect of the saturable layer. This effect may be tuned, e.g. by changing the temperature of the substrate. Saturable Bragg Reflectors are described in detail in U.S. Pat. No. 5,627,854 issued to Wayne Knox, which is incorporated herein by reference.
SBRs provide a nonlinear saturation response for mode-locking lasers. A mode-locked laser typically comprises a lasing medium such as Nd:YAG disposed in an optical cavity. A pump source, such as a diode laser pumps the lasing medium to produce stimulated emission of light. An SBR typically serves as one of the reflectors for the laser cavity. Optical pulses produced by the laser tend to have higher fluence and, therefore, lower loss at the SBR. Consequently, such a laser tends to favor pulse formation at high fluence.
The properties of the substrate of the SBR affect the performance of the laser. For example if the substrate transmits light, some light may reflect off both the front and back surfaces. Light reflecting from the back surface produces feedback that interferes with light reflecting from the front surface and affects the performance of the SBR and mode-locked laser. Such interference effects have either been ignored in the prior art, because the front surface is much more reflective than the back surface, or viewed as detrimental to laser operation. Sometimes, SBRs are manufactured with substrates having polished back surfaces. Where the effect was not negligible, the prior art removed the effect by sanding the back surface to roughen it, thereby making it less reflective. The effect has also been observed to depend on the type of glue or solder used to attach the substrate to a supporting plate.
The pulses produced by a laser may be altered using an etalon. Etalons have been previously used to control the spectrum in mode-locked lasers. An etalon uses two parallel reflecting surfaces separated by a known distance. Interference of light reflected or transmitted from the two surfaces reinforces certain wavelengths of light, while tending to cancel out others. It is well known to use an etalon to affect the spectrum of a laser to lengthen or shorten optical pulses in a mode-locked laser. Alternatively the etalon may be used to optimize the relation between the temporal and frequency domains of the laser spectrum or adjust the distribution of power amongst the various modes of the laser. Prior art etalons have been separate discrete elements. Consequently, these etalons are difficult to implement in a mode-locked laser because they are difficult to align properly. Etalons must generally be temperature and vibration stable. In short, an etalon usually adds to the complexity of construction and cost of a mode-locked laser.
A variety of passive mode-lockers based on film technology have been developed or proposed recently. These are classified in the literature as saturable Bragg reflectors (SBR), semiconductor saturable absorbing mirrors (SESAM), anti-resonant Fabry-Perot saturable absorbers (AFPSA), and hybrid saturable reflectors. AFPSA""s were the first devices to use nonlinear semiconductor saturable absorption as a passive mode-locking technique. [Keller, 1992, Keller, 1996, Keller, 1993] An AFPSA device has a substrate upon which is applied a Bragg mirror stack, a saturable absorber in a buffer layer, and a surface coating. The thickness of the buffer layer is designed for anti-resonance. The surface coating is typically a partial reflector to control the field intensity in the absorber region, although variants include an anti-reflection coating or no coating at all in which case the surface reflectivity was determined by the Fresnel reflectivity of the interface.
A hybrid saturable reflector consists of a substrate with an absorbing layer grown on the back surface (rather than on the top surface), then covered by a dielectric or metal mirror. [Alcock, 1999] In this approach the laser field passes through the substrate but the interaction is benign so the only purpose of the substrate is to support the saturable absorber and mirror structure. No etalon effect is employed; in fact, the front surface of the substrate is AR coated to avoid any possibility of etalon effect. Alternatively the front surface could be left uncoated or coated with a partial reflector to introduce etalon effects.
There is a need, therefore, for a low cost, simple, tunable, mode-locked laser having a saturable reflector that incorporates an etalon to control the laser spectrum.
Accordingly, it is a primary object of the present invention to provide a saturable Bragg reflector having integral etalon tuning. It is a further object of the invention to provide a mode-locked laser with low cost, reduced complexity, and improved performance.
These objects and advantages are attained by a Saturable Bragg Reflector apparatus comprising a substrate having a front surface and a back surface, and a Bragg stack, having a saturable absorber layer, attached to the front surface. At least one of the front and back surfaces has been modified to enhance an etalon effect due to interference of light reflecting from the front and back surfaces. Either or both of the surfaces may be modified, for example, by polishing or coating. The apparatus may also include means for adjusting an optical thickness of the substrate to tune the etalon effect. Such means may comprise a temperature control element, such as a heater, coupled to a temperature controller. The inventive apparatus may be incorporated into a mode-locked laser. Tuning the substrate etalon effect implements an inventive method for tuning a saturable reflector. The etalon tuning optimizes a relation between temporal and frequency domains of radiation incident on the SBR.
In embodiments of the present invention, the SBR device is monolithically integrated with a Fabry-Perot etalon fabricated in the same substrate. A temperature control circuit tunes and stabilizes the device operating point. It is the interaction of the etalon modes with the laser spectrum and the saturable absorber temperature which causes changes to the pulse width. The etalon typically has a free spectral range (FSR) of order 1 GHz or greater. The etalon thickness is generally large enough to give an FSR on order of the laser linewidth.
A specific embodiment of the invention comprises a device having a GaAs wafer substrate with structures on the front and back surfaces. On the front surface is a saturable Bragg reflector, consisting of a Bragg reflector stack and a quantum-well nonlinear absorption layer. On the back surface is a reflector, which forms an etalon with the substrate and Bragg reflector. Thus the device integrates a passive mode-locker with an etalon which modifies the spectral content of the light to alter the pulse length produced by the mode-locked laser. In particular, the device is used to build a mode-locked laser with a flattened gain spectrum by suppressing the gain peak, thereby increasing the bandwidth and shortening the pulse length.